Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas accepting ink are the printing image areas and the ink-rejecting areas are the background areas.
In the art of photolithography, a photographic material is made image-wise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed (positive-working) areas on a hydrophilic background. Common photographic materials employed for making a lithographic printing plate are materials based on a photosensitive polymer or are silver salt diffusion transfer materials.
Silver salt diffusion transfer materials offer the advantage of having a high speed in a broad spectral range, whereas common photosensitive polymer materials are only sensitive to light of a narrow spectral range, commonly between 300 and 500 nm. Furthermore, photosensitive polymers do not offer the speed of a silver halide material.
Silver salt diffusion transfer materials require the use of liquid processing baths. As a consequence, the formation of a lithographic printing plate by means of such materials takes much time and requires much labour. The chemicals used in the wet processing are harmful to the human body and they soil the cloths and hands of the operators.
In GB-P 1,454,271 a method of making a lithographic printing plate in a dry manner has been described, which method comprises image-wise exposing to actinic radiation the sensitive layer of a photosensitive heat-processable photographic material, which layer contains a binder and, as part of an image-forming system, a light-insensitive reducible metal salt of a straight-chain fatty acid having at least 10 carbon atoms, heating the layer to develop a first image therein by reduction of the metal salt and thereby liberate fatty acid, and holding the layer in contact with a receiving sheet e.g. a hydrophilic grained anodized aluminium foil, whilst being held at a temperature above the melting point of the fatty acid so that fatty acid-containing material transfers from the developed areas to the receiving sheet to produce an oleophilic transfer image. However an important disadvantage is that the handling of said imaging element is time-consuming and cumbersome.